1. Field of the Invention
The present invention relates generally to an adjustment method for a magneto-optical memory device and a magneto-optical memory device, and more particularly to an adjustment method for a magneto-optical memory device and a magneto-optical memory device for storing data using light beams and magnetic fields.
2. Description of the Related Art
Magneto-optical memory devices, such as magneto-optical disk devices, store data by simultaneously focusing or projecting light emitted from a light source through an objective lens onto a recording surface of a magneto-optical disk comprising the recording medium and applying an external magnetic field to the position on the disk onto which the light beam is projected or focused. An overwrite system, involving magnetic field modulation recording using a magnetic head capable of reduced magnetic field switching time, has been proposed as one method of reducing the data storage time.
As the recording density of the above-described magneto-optical disk increases it becomes increasingly necessary to position the light beam and magnetic field accurately so as to reduce data storage time.
FIG. 1 is a diagram showing the structure of an essential part of a conventional magneto-optical disk device. As shown in the diagram, the conventional magneto-optical disk device comprises a light source 2 provided on an optical head 1. Light emitted from the light source 2 is emitted from the optical head 1 and supplied to a reflecting mirror 3.
The reflecting mirror 3 deflects the light supplied from the optical head 1 at an angle of 90.degree.. An objective lens 4 focuses the light supplied from the reflecting mirror 3 on a magneto-optical disk 5.
The objective lens 4 is supported by a lens holder 6. The lens holder 6 is movably supported by an actuator (not shown in the diagram) so as to be movable in a focusing direction (arrow A) and a tracking direction, that is, in a direction of a radius of the magneto-optical disk 5 (arrow B).
A magnetic head 7 is provided on a side opposite the objective lens 4, with the magneto-optical disk 5 positioned between the magnetic head 7 and the objective lens 4. The magnetic head 7 is attached to a suspension arm 8 and the suspension arm is attached to a carriage 9. The carriage 9 is movably supported by an actuator (not shown in the diagram) so as to be movable in the direction of the radius of the magneto-optical disk 5 (arrow B).
It should be noted that, because the magnetic head 7 is fixedly mounted on the carriage 9, the relative positions of the objective lens 4 and the magnetic head 7, that is, the positions of the objective lens 4 and the magnetic head 7 with respect to each other, do not change even when the carriage 9 is moved.
It should also be noted that the carriage 9 is fixed in such a way that a position of a spot 10 on the magneto-optical disk 5 on which the objective lens focuses light and a position of the magnetic head 7 are roughly identical with respect to a hypothetical horizontal plane parallel to the recording surface of the magneto-optical disk 5.
In recent years, the size of the magnetic head 7 has been reduced in order to reduce the switching time of an external magnetic field. As a result, the effective range of the external magnetic field generated by passing an electric current through a coil of a magnetic head element within the magnetic head 7 has been narrowed drastically to approximately 100-200 .mu.m. Given the narrowness of this effective range, even a slight misalignment of the light spot 10 and the magnetic head 7 results in an inability to generate an effective magnetic field for recording and playing back data.
For this reason, then, the positions of the light spot 10 and the magnetic head 7 are adjusted when the magnetic head 7 is mounted on the carriage 9 during manufacture. A description will now be given of one conventional method for adjusting the position of the light spot 10 and magnetic head 7.
FIGS. 2(A) and 2(B) are diagrams of the structure of an essential part of a conventional magneto-optical disk device. FIG. 2(A) is a cross-sectional view of the magnetic head 7 and FIG. 2(B) is a cross-sectional view of the optical system.
As shown in FIG. 2(A), the magnetic head 7 comprises coils 11 and a magnetic pole portion 12. The center magnetic pole 13 of the magnetic pole portion 12 is designed so that a surface of the center magnetic pole 13 opposing the magneto-optical disk 5 has a reflection factor that is greater than that of the adjacent magnetic poles 14 and the surrounding coils 11.
As shown in FIG. 2(B), when adjusting the positions of the light spot 10 and the magnetic head 7 a transparent substrate 15 having the same characteristics as those of a typical magneto-optical disk is positioned between the magnetic head 7 and the objective lens 4. Light emitted from the light source passes through a beam splitter 16 and the objective lens 4 and is trained on the transparent substrate 15. The light so trained on the transparent substrate 15 passes through the transparent substrate 15 and reaches the magnetic head 7. The light that reaches the magnetic head 7 is trained on the magnetic pole portion 12.
The light trained on the magnetic head 7 is reflected by the magnetic head 7 and supplied to the beam splitter 16 via the transparent substrate 15 and the objective lens 4. The beam splitter 16 supplies the light reflected by the magnetic head 7 to a photodetector 17. The photodetector 17 outputs an output signal in response to the amount of light reflected from the magnetic head 7.
It should be noted that a reflecting layer is provided on the center magnetic pole 13 of the magnetic head 7 so that the center magnetic pole 13 has a reflection factor that is greater than that of adjacent members of the magnetic head 7, and so the amount of light reflected is at a maximum amount at a center core 13, that is, a center of the magnetic field generated at the magnetic head 7.
FIGS. 3(A), 3(B) and 3(C) are diagrams for explaining the operation of the conventional position adjustment method, the structure of which has been described above. FIGS. 3(A) and FIG. 3(C) show a state in which the center of the magnetic head 7 and the axis 18 of the light beam 10 do not match, while FIG. 3(B) shows a state in which the center of the magnetic head 7 and the axis 18 of the light beam 10 do match.
FIG. 3(A) shows a state of the center magnetic pole 13 of the magnetic head 7 deviating from the axis 18 of the light beam 10 in the direction of the arrow C1. In this state, the axis 18 of the light beam 10 is trained not on the reflecting layer of the center magnetic pole 13 of the magnetic head 7 but on an adjacent area of the magnetic head 7. As a result, the amount of light reflected decreases and, accordingly, the output of the photodetector 17 also decreases.
Similarly, FIG. 3(C) shows a state of the center magnetic pole 13 of the magnetic head 7 deviating from the axis 18 of the light beam 10 in the direction of the arrow C2. As with the state shown in FIG. 3(a) described above, in this state, too, the axis 18 of the light beam 10 is trained not on the reflecting layer of the center magnetic pole 13 of the magnetic head 7 but on an adjacent area of the magnetic head 7. As a result, the amount of light reflected decreases and, accordingly, the output of the photodetector 17 also decreases.
FIG. 3(B) shows a state in which the magnetic head 7 and the axis 18 of the light beam 10 do match. In this state, the axis 18 of the light beam 10 is aligned with the center magnetic pole 13 of the magnetic head 7 and, thus, the amount of light reflected is at a maximum amount.
FIG. 4 is a diagram showing the relation between the amount of light reflected and the misalignments of the axis 18 of the light beam 10 and the center magnetic pole 13 of the magnetic head 7, in a conventional position adjustment method.
As shown in FIG. 4, when the center magnetic pole 13 of the magnetic head 7 and the axis 18 of the light beam 10 match, the amount of light reflected from the magnetic head 7 attains a maximum value. Accordingly, the output signal of the photodetector 17 attains a maximum value indicated by the solid line shown in FIG. 4.
However, when the axis 18 of the light beam 10 and the center magnetic pole 13 of the magnetic head 7 are in a state of misalignment as described above and as shown in FIGS. 3(A) and FIG. 3(C), then the amount of light reflected from the magnetic head 7 decreases to a level less than that of the maximum value, such lesser level being indicated by the dotted line shown in FIG. 4. Accordingly, the position of the magnetic head 7 is adjusted so that output of the photodetector 17 attains a maximum value.
A description will now be given of another conventional position adjustment method with reference to FIG. 5, which is a diagram for explaining the operation of another conventional position adjustment method. Items identical to those shown in FIG. 2 have been given identical numbers and a description thereof will be omitted for the sake of convenience.
In this position adjustment method, either an actual magneto-optical disk or an equivalent test disk 19 (hereinafter magneto-optical disk 19) is used in play mode to adjust the relative positions of the magnetic head 7 and the objective lens 4. The light reflected from the magneto-optical disk or test equivalent 19 is split by the beam splitter 16 and supplied to a Wollaston polarizing prism 20, where a reproduced signal component is extracted, focused by a focusing lens 21 and supplied to a photodetector 22.
The photodetector 22 obtains from the light supplied from the focusing lens 21 a reproduced signal in response to data previously recorded on the magneto-optical disk 19. The reproduced signal obtained by the photodetector 22 is monitored and the relative positions of the magnetic head 7 and the objective lens 4 are adjusted so that the signal level attains a predetermined level.
FIGS. 6(A), 6(B) and 6(C) are diagrams showing a third conventional position adjustment method. FIGS. 6(A) and 6(C) show states in which the magnetic head 7 and the optical head that focuses the light beam 10 deviate from an optimal position, while FIG. 6(B) shows a state in which the magnetic head 7 and the optical head that focuses the light beam 10 are optimally positioned.
In FIG. 6(A), the magnetic head 7 deviates from the axis 18 of the light beam 10 in the direction of arrow C1, that is, the center of the magnetic field generated at the magnetic head 7 is in a state of deviation from the axis 18 of the light beam 10 in the direction of the arrow C1. In this condition, the axis 18 of the light beam 10 and the center of the magnetic field generated at the magnetic head 7 are misaligned and, as a result, the intensity of the magnetic field decreases at the portion of the light beam 10 focused on the magneto-optical disk 19.
Similarly, in FIG. 6(C) the magnetic head 7 deviates from the axis 18 of the light beam 10 in the direction of arrow C2, that is, the center of the magnetic head 7 deviates from the axis 18 of the light beam 10 in the direction of arrow C2. In this condition, the axis 18 of the light beam 10 deviates from the center of the magnetic head 7 and, as a result, the intensity of the magnetic field operating on the portion of the light beam 10 focused on the magneto-optical disk 19 decreases.
In FIG. 6(B) the center of the magnetic head 7 and the axis 18 of the light beam 10 are in a state of alignment. At this time, the magnetic head 7 and the optical head 7 are optimally positioned.
FIG. 7 is a diagram showing the relation between the amount of light reflected and the misalignments of the axis 18 of the light beam 10 and the center magnetic pole 13 of the magnetic head 7, in a third conventional position adjustment method.
As shown in FIG. 6(B), in a state in which the center of the magnetic head 7 and the axis 18 of the light beam 10 are in alignment, the intensity of the magnetic field applied from the magnetic head 7 to the position at which the light beam 10 is focused on the magneto-optical disk 19 is at a maximum intensity. Accordingly, the resulting reproduced signal reaches a maximum amplitude as shown by the solid line in FIG. 7.
Additionally, as shown in FIGS. 6(A) and FIG. 6(C), when the center of the magnetic field generated at the magnetic head 7 and the axis 18 of the light beam 10 are in a state of misalignment, the intensity of the magnetic field applied from the magnetic head 7 to the position at which the light beam 10 is focused on the magneto-optical disk 19 decreases and, accordingly, the amplitude of the resulting reproduced signal shrinks as shown by the broken line in FIG. 7.
In the third conventional position adjustment method described above, the reproduced signals output as shown in FIG. 7 are detected and the position of the magnetic head 7 adjusted so that the reproduced signal attains a maximum amplitude.
It should be noted, however, that reducing the switching time of the external magnetic field is useful for reducing data recording/reproducing time. It is particularly important to reduce the switching time in the case of the magnetic field modulation recording method. However, reducing the switching time of the external magnetic field, although it can be accomplished with compact magnetic heads, results in a magnetic field of reduced effective range.
The relative positions of the magnetic head 7 and the objective lens 4 are subject to stress, for example minute deformations of the structural members due to heat or fluctuations in the effective range of the external magnetic field, that can change the relative positions of the magnetic head 7 and the objective lens 4 even after the relative positions of the magnetic head 7 and the objective lens 4 have been adjusted and fixed. Accordingly, it is desirable to fix the positions of the light spot and peak strength of the external magnetic field as accurately as possible and to maintain those positions after adjustment and fixing.
However, fine adjustments to the position of peak strength of the magnetic field generated at the magnetic head 7 cannot be made with the conventional position adjustment method shown in FIG. 3.
Similarly, accurate alignment of the light spot and the position of peak strength of the external magnetic field cannot be made with the conventional position adjustment method shown in FIG. 6 because the reproduced signal level attains a level greater than a predetermined level when an external magnetic field is applied so that a magneto-optical signal of a level greater than a certain level is detected at the photodetector 36.
Additionally, when adjusting the positions of the light spot and the magnetic head of the conventional magneto-optical disk apparatus, an external pressure 13 is exerted on a magnetic head fixing portion 37 with respect to a mounting surface of the carriage and, after adjustment, a screw or similar fixing means is used to fixedly mount the magnetic head fixing portion 37 on the mounting surface of the carriage. As a result of this operation, the position of the magnetic head shifts slightly as the screw or similar fixing means is tightened, thus slightly altering the alignment of the magnetic head relative to the light spot.